Such a set of blades is described in German Pat. No. 893,649.
It is known that in a given passage, at points which are sufficiently far from the walls of the passage, the stream lines follow paths which are substantially parallel to the walls of the passage formed by the concave and convex surfaces of the blades. At all points along the path, the centrifugal force which is exerted on a particle is balanced by the pressure forces. The result of this is, generally, that the concave surface of the blade is subjected to a higher pressure than is the convex surface of the blade which delimits the passage.
It is also known that in the boundary layer near the lower plate and upper plate, the speed of the fluid is low; it follows that the pressure forces are no longer balanced and the stream lines are curves perpendicular to the isobars and follow paths of considerable slippage in each passage from the concave surface to the convex surface as is well known to the person skilled in the art (see, for example, the article in the November 1941 French issue of the Brown Boveri review--see p. 356 to 361 and, in particular, FIGS. 2 and 3).
The slippage generates a counter-clockwise eddy against the upper plate of the passage and a clock-wise eddy against the lower plate as seen by an observer placed downstream from the set of blades.
These disturbances cause important losses known as secondary losses and the smaller the ratio between the height of the blades and the chord, the more the efficiency of a set of blades is reduced.
German Pat. No. 893,649 provides for a reduction in secondary losses by providing ducts which discharge into the passages and have openings located in the mid portion of the upper plate at an equal distance from the concave surface of one blade and the concave surface of the blade next to it which delimit a passage. The diameter of said duct is large enough (with respect to the dimensions of the passage) to allow the boundary layer to be aspirated.
Aspirating said boundary layer makes it possible to reduce disturbance greatly, but it does not reduce secondary losses as much; indeed, the aspirated fluid no longer has any effect in the passage from which it was aspirated and it causes disturbance in the fluid which passes through the following set of blades through which it is reinjected.
In the invention, on the contrary, only a small portion of the boundary layer is aspirated to prevent slippage thereof and thereby to make the fluid of the boundary layer useful.